Get online support
Hi @Sun2136, he can login to his TPG online account and update his payment details there.
If your grandpa has NBN service, here's how he can update it, https://community.tpg.com.au/t5/Accounts-and-Billing/Using-TPG-My-Account-amp-Online-Account-Info/m-...
For Home Wireless Broadband service, check out https://community.tpg.com.au/t5/Accounts-and-Billing/How-To-Update-Payment-Details-New-TPG-My-Accoun...
Alternatively, he can contact our customer service at 1300 993 011.